The use of curved transparencies with rabbeted edges is called for in certain specialized glazing applications such as aircraft landing light covers. The rabbeted edges permit the glass sheet to be mounted with its outer surface flush with the outer surface of the aircraft, and the curved shape conforms to the overall streamlined shape of the surrounding portions of the aircraft. The need for such transparencies has heretofore been fulfilled by clear plastic sheet materials, despite their inferior durability, because it was not believed practical to fabricate such a shape from glass. It would be highly desirable, therefore, to be albe to make curved, rabbeted glass sheets to replace plastic in such applications.
A method of grinding rabbeted edges on glass sheets has been disclosed in U.S. applicaton Ser. No. 745,705, filed on Nov. 29, 1976, by Joseph B. Kelly and assigned to the assignee of the present invention, PPG Industries, Inc. However, bending a rabbeted glass sheet to the desired curvature is an additional problem for which there was no solution in the prior art. Difficulty in bending such a rabbeted glass sheet arises from the fact that when the sheet is heated to a temperature suitable for bending, the rabbeted edge portions, being of reduced thickness, become softer and easier to bend than the main body of the glass sheet. Thus, when conventional bending techniques are employed, the softened thin rabbeted portions are bent too quickly or too sharply to permit adequate control of the final shape. In gravity sag bending, where a peripheral shaping rail is used to support the glass, the rabbeted edges become too soft to support the weight of the glass sheet on the rail. If the glass is supported inwardly from the rabbeted portion, the unsupported rabbeted portion then droops in the wrong direction. Press bending offers little improvement since the press faces cannot be brought to bear against opposite sides of the thin rabbeted portions, and therefore the curvature in the rabbeted portions cannot be positively effected. Attempts to provide the press face with a stepped contour to match the contour of the rabbeted glass have been found to be unsatisfactory. While the correct shape could be imparted to the glass sheets, including the rabbeted edge portions, it was found that cracks were formed along the edges of the sheets which seriously reduced their strength. It has now been theorized that this problem is caused during the initial moments of contact between the glass and the press face, at which time only the edge portions of the glass contact the female press face. Because the rabbeted portion is soft and easily bent, this initial contact causes the rabbeted portion to become severely bent temporarily. As the press members close, the severity of the bend at the edges is reduced as the center portion of the glass begins to bend. It is believed that this severe initial bending action in the rabbeted areas sets up undesirable stress patterns in that portion of the glass which cause weakening of the edges.